The present disclosure relates to a resistance-change memory device in which data is written using a spin transfer effect caused by current injection.
In accordance with the recent significant widespread use of data communication apparatuses, in particular, small personal apparatuses such as mobile terminals, there has been a demand for memories, logic devices, and the like constituting these apparatuses to have higher performance including a higher degree of integration, a higher speed, and lower power consumption. In particular, nonvolatile memories are considered to be indispensable components for realizing high performance apparatuses.
Semiconductor flash memories and ferroelectric nonvolatile memories (FeRAMs) have been put to practical use as such nonvolatile memories. At present, active research and development activities are underway to achieve even higher performance.
Recently, magnetic random access memories (MRAMs) utilizing a tunnel magnetoresistance effect have significantly advanced through extensive development, and have drawn much attention as a new type of nonvolatile memory that uses a magnetic material (see, for example, J. Nahas et al., IEEE/ISSCC 2004 Visulas Supplement, p. 22).
Here, the principle of operation of an MRAM, which is closely related to the present invention is briefly described.
An MRAM is a magnetic data storage device having a structure provided with regularly arranged minute memory elements made of a magnetic material and wiring arranged so as to allow access to each of the memory elements.
When currents are passed through both a conductor line (word line) and a read conductor line (bit line) arranged above or below the magnetic memory elements, a combined current magnetic field is induced. Writing of data into an MRAM is performed by controlling the magnetization of each magnetic element using the combined current magnetic field.
Generally, “0” or “1” is stored in accordance with the direction of magnetization. Typical methods of rewriting the data in a memory element include a method utilizing asteroid characteristics (see, for example, Japanese Unexamined Patent Application Publication No. 10-116490). There also exists a method utilizing switching characteristics (see, for example, U.S. Patent Application Publication No. 2003/0072174).
Reading of data is performed by selecting a particular memory cell via a device such as a transistor and obtaining information about the direction of magnetization as a voltage signal utilizing a galvanomagnetic effect.
A memory cell structure which has been proposed is a structure including a three-layer junction (magnetic tunnel junction: MTJ) having a ferromagnetic layer, an insulating layer, and a ferromagnetic layer. This structure is called an MTJ structure hereinafter.
In an MTJ structure, one ferromagnetic layer is used as a fixed reference layer having a fixed magnetization direction, and the other as a recording layer (free layer). In this manner, the magnetization direction of the recording layer is made to correspond to a voltage signal utilizing the tunnel magnetoresistance effect, in an MTJ structure.
An MRAM allows “0” and “1” data based on switching of the polarity of magnetic material to be rewritten at high speed and almost an infinite number of times (1015 times or more). This is the primary feature of an MRAM compared with other nonvolatile memories.
On the other hand, substantial power is necessary in an MRAM because a current of several to several tens of mA is passed through the wiring. In addition, miniaturization of the memory cells is difficult because both word lines for recording and read lines for reading are necessary in an MRAM. Furthermore, when the MTJ is decreased in size, a stronger magnetic field is necessary for switching in an MRAM, and hence, resulting in a disadvantage in terms of scaling for miniaturization from the viewpoint of power consumption.
As one solution, research has been conducted regarding a recording method that does not use a current magnetic field, and research on spin transfer switching, among others, is ongoing (see, for example, U.S. Pat. No. 5,695,864).
A memory element using spin transfer switching employs an MTJ structure similarly to an MRAM. However, spin transfer switching utilizes the fact that spin polarized electrons passing through a magnetic layer having a certain fixed magnetization direction exert torque on a free layer when the electrons enter the free layer. Specifically, the magnetization of the free layer switches when a current equal to or more than a threshold is passed.
Writing of “0” or “1” is controlled by changing the polarity of the current.
The absolute value of a current necessary for this switching is several mA or less for an element that is on a scale of about 0.1 μm, and moreover, decreases with the volume of the element. In this respect, a spin transfer switching memory element has an advantage in terms of decreasing the scale of an element.
In addition, a spin transfer switching memory element has an advantage in that the memory cell is simplified because the word lines for recording necessary in an MRAM are not necessary.
In reading of data, the tunnel magnetoresistance effect is used similarly to MRAM.
Hereinafter, an MRAM utilizing spin transfer switching is called a spin transfer random access memory (SpRAM). Likewise, a spin-polarized electron flow causing spin transfer is called a spin injection current.
An SpRAM is considered to be a highly promising nonvolatile memory which realizes lower power consumption and larger capacity while keeping the advantages of MRAM in terms of high speed and being able to be subjected to an almost infinite number of rewriting operations.
In an already proposed SpRAM, data rewriting between “0” and “1” is performed by changing the polarity of a spin injection current.
However, the result of magnetic polarity switching may not necessarily be determined only by the polarity of the spin injection current due to instability involved in the spin transfer switching phenomenon.
In an SpRAM, in addition to the magnetization states respectively corresponding to “0” and “1” data, there exists a quasi-stable state observed only when a spin injection current is caused to flow. This instability in the magnetization switching result is caused by a phenomenon in which once the magnetization is trapped in the quasi-stable state, the magnetization state after the current is stopped becomes unstable.